Microphone arrays have been widely studied because of their effectiveness in enhancing the quality of the captured audio signal. The use of multiple spatially distributed microphones allows spatial filtering, filtering based on direction, along with conventional temporal filtering, which can better reject interference signals. This results in an overall improvement of the captured sound quality of the target or desired signal.
Beamforming operations are applicable to processing the signals of a number of arrays, including microphone arrays, sonar arrays, directional radio antenna arrays, radar arrays, and so forth. For example, in the case of a microphone array, beamforming involves processing output audio signals of the microphones of the array in such a way as to make the microphone array act as a highly directional microphone. In other words, beamforming provides a “listening beam” which points to, and receives, a particular sound source while attenuating other sounds and noise, including, for example, reflections, reverberations, interference, and sounds or noise coming from other directions or points outside the primary beam. Pointing of such beams is typically referred to as beamsteering. A typical beamformer employs a set of beams (i.e., beamforming) that cover a desired angular space range in order to better capture the target signal.
Various microphone array processing algorithms have been proposed to improve the quality of the target signal. The generalized sidelobe canceller (GSC) architecture has been especially popular. The GSC is an adaptive beamformer that keeps track of the characteristics of interfering signals and then attenuates or cancels these interfering signals using an adaptive interference canceller (AIC). This greatly improves the target signal, the signal one wishes to obtain. However, if the actual direction of arrival (DOA) of the target signal is different from the expected DOA, a considerable portion of the target signal will leak into the adaptive interference canceller, which results in target signal cancellation and hence a degraded target signal.
In order to avoid target signal cancellation, a number of methods have been proposed to make such a side lobe cancelling system more robust to steering vector errors. A robust GSC (RGSC) has been proposed which uses an adaptive blocking matrix with coefficient-constrained adaptive filters, which prevents the target signal from leaking into the adaptive interference canceller. More specifically, the RGSC takes the outputs from the microphone array and inputs these into a fixed beamformer that directs a beam toward the desired signal. The beamformer output contains the enhanced signal originating from the pointed direction which is used as a reference signal by the adaptive blocking matrix. The adaptive blocking matrix adaptively subtracts the reference signal from each channel input and provides the interference signals. In order to suppress only those signals that originate from a specific tracking region, the adaptive filter coefficients of the adaptive blocking matrix are constrained on the maximum allowed deviation between the expected DOA and the actual DOA. The interference signals obtained from the adaptive blocking matrix are passed to an adaptive interference canceller which removes the interference signal components from the beamformer output. This minimizes undesirable target signal cancellation. In addition to this, the AIC uses norm-constrained adaptive filters that can further improve the robustness against target signal cancellation. This adaptive beamformer is robust to DOA errors without significant degradation in the interference rejection capability. A similar RGSC has been implemented in frequency-domain, with a comparable performance at a lower computational cost.
Although the above-described RGSCs are good at rejecting directional interference signals (such as “jammer” signals), their noise suppression capability is not satisfactory under the conditions of practically isotropic ambient noise. Therefore these algorithms are not particularly suitable for noisy environments such as the automotive environment or reverberant environments such as offices or conference rooms.